With the gauge removed Looking at the rear you will see two holes with small screw driver slots. See fig 5) These are the adjusters for setting the E and F calibration points.
As you look at the front of the gauge, the adjuster behind E to ½ , adjusts the E(Empty) pointer position. The adjuster behind the 1/2 to F adjusts the F(Full) pointer position.
Next on the rear of the gauge you will see two wires. A light green/dark green tracer wire that connects to the voltage stabilizer(10v) and a dark green/brown tracer wire that comes from the sender unit. Remove the dark green/brown wire from the gauge and in its place attach one end of the 222 Ω (E value) to the gauge. Connect the other end of the resistor to the Black earth wire.
OK we are ready to start calibrating. However two things I learned and to be aware of before making any adjustment.
The screw driver adjustments, particularly the Zero are very crude, the slightest movement can cause quite a large change in pointer movement.
Secondly the two adjustments (Empty and Full) interact with each other.
One last thing before we start. Either make sure the gauge housing is not connected to or touching ground or ensure the blade of your screw driver is insulated as the adjusters are connected to the gauge electrical circuit and could damage the stabilizer if shorted out. .
Turn on the ignition and allow the gauge to read. Remember the gauge is slow to respond so allow 20 – 30 seconds between any adjustments. With the 222 Ω resistor, the gauge should read E but in my case was reading ¼ full. So bearing in mind the above I opted for the ½ and ½ approach. Using a small screwdriver I turned the Empty adjuster to position the needle so it now showed half way between ¼.and E.(Half the original reading) Replacing the resistor with the 20 Ω value to represent a F (full tank). (This is where my little switch box made this easier) allowing 20-30 seconds for it to stabilize my gauge now read just over ¾ full. So with my screw driver in the FULL adjustment hole adjusted the needle to read half way between the original reading and F. Then replacing the resistor again with the empty value (222 Ω) allowed for the needle to stabilze at the zero end. Again making an adjustment to half the error then repeating with Full resistor value. Repeat this until the gauge gives the correct readings at Full and Empty with one final check at ½ full. My gauge seems to have some hysteresis as when going from F to ½ the gauge read exactly ½ (see fig 6), but when going from E to ½ it read just under the ½ mark. In all this took me around 30 mins.
Turn off the ignition and reassemble the gauge into the dash.